President Barack Obama on Tuesday slapped down Donald Trump's claim that the 2016 presidential race is rigged, telling the Republican nominee to "stop whining" and get on with his campaign.

In language usually used to scold a moody teenager, Obama discarded diplomatic decorum, skewering the mogul from the Rose Garden in front of visiting Italian Prime Minister Matteo Renzi.

Trump has ramped up conspiracies about America's election system as his poll numbers have plummeted in the wake of sexual assault allegations against him.

He trails around seven points nationwide and bookmakers in Europe -- where political betting is legal -- have already begun to pay out on a Clinton win.

But the White House is increasingly concerned that Trump and his supporters will not recognize the election's outcome, plunging the country into a political crisis.

According to a poll by Politico and Morning Consult, 41 percent of American voters, including 73 percent of Republicans, now believe the vote could actually be stolen from Trump.

"I have never seen in my lifetime or in modern political history any presidential candidate trying to discredit the elections and the election process before votes have even taken place. It's unprecedented," Obama said.

"If, whenever things are going badly for you and you lose, you start blaming somebody else? Then you don't have what it takes to be in this job," he added.

Addressing Trump's allegations of "large scale voter fraud," Obama said "there's no evidence that that has happened in the past, or that there are instances in which that will happen this time."

"I'd advise Mr Trump to stop whining and go try to make his case to get votes," Obama told a news conference.

- 'Final shot' -
The withering riposte comes on the eve of the third and final presidential debate.

With three weeks until the November 8 election, this may be Trump's last chance to make a positive mark on millions of voters.

Campaigning in Colorado on Tuesday, a slightly subdued Trump indicated his scorched-earth tactics would continue.

"We've only just begun to fight, believe me," he said. "This is our final shot, folks."

Polls giving Clinton the lead were inaccurate, he said. In fact, "they're sort of good" for the Trump campaign.

"We are going to have one of the greatest victories in political history," he predicted, excoriating the media for being "an extension of the Hillary Clinton campaign."

"The press has created a rigged system and poisoned the minds of the voters," he said.

"Either we win this election or we lose the country."

Trump doubled-down on his vote rigging claims, saying that "noncitizens" might decide the election and "voter fraud is all too common" in cities such as Philadelphia, Chicago and St. Louis -- which have large African-American populations.

"Take a look at some of these cities where you see things happening that are horrendous," Trump said, going on the defensive, "and if you talk about them, they say bad things about you. They call you a racist."

"By the way, speak of that, nobody will do more for the African-American citizens of this country than Donald Trump. Nobody. Nobody," Trump said, referring to himself in the third person.

Experts and Republican elected officials have denounced Trump for his accusations of voter fraud, prompting the nominee to turn on his own party.

He went a step further on Tuesday, calling for term limits for the US Senate and House of Representatives.

But Clinton's campaign believes Trump's hot rhetoric has helped the Democrats -- not only by galvanizing supporters but also shifting undecided voters to their camp.

"We know that he thought that strategy of scorched earth would depress our vote, but if anything, we have found that it's helped to motivate our voters," Clinton communication's director Jennifer Palmieri said.

Democrats are beginning to target traditionally Republican states in a bid to run up the score and help win legislative races that could decide who controls Congress.

On Thursday, First Lady Michelle Obama will campaign in Arizona, which has not voted Democratic since Bill Clinton's landslide win against Bob Dole in 1996.